Comparison of different speciation techniques to measure Zn availability in hydroponic media

Abstract

Four analytical techniques are compared: AGNES (Absence of Gradients and Nernstian Equilibrium Stripping), LASV (Anodic Stripping Voltammetry with Linear stripping), DGT (Diffusive Gradients in Thin films) and PIM (Polymer Inclusion Membranes). These techniques have been designed to provide the free ion concentration or a labile fraction, complementarily contributing to an integrated description of speciation and availability. Their simultaneous application to the determination of free Zn concentrations or labile fluxes in seven solutions of a hydroponic medium reveals characteristics of each technique and correlations between their results. All dynamic results can be interpreted in terms of a general theoretical framework on fluxes. Indeed, in techniques under diffusion-limited conditions in the sample, the flux can be split into the free contribution (linearly proportional to the free fraction), plus the contribution of the complexes (where mobility, lability and abundance of complexation are intertwined). A methodology to compute lability degrees is developed. Measurements with PIM devices confirm that diffusion in the sample solution is not rate limiting, so its flux is proportional to the free metal in the donor solution. A proportionality between the responses of any given two techniques is observed, which suggests that, for the low ligand-to-metal concentration ratios used in the present work, any of these techniques would correlate similarly with uptake, toxic or nutritional measurements.

New methodology to measure low free indium (III) concentrations based on the determination of the lability degree of indium complexes. Assessment of In(OH)3 solubility product

“…However, AGNES (Absence of Gradients and Nernstian Equilibrium Stripping) [13,14] was specifically designed to measure free metal ion concentrations for amalgamating ions with negative standard redox potentials. AGNES has dealt with Zn, Cd and Pb, in a range of systems including seawater [15], river water [16][17][18], estuarine water [19], soil extracts [16], wine [20], hydroponic media [21] as well as solutions containing dissolved organic matter [22,23], and nanoparticles [24][25][26][27][28][29][30][31][32]. Very recently, AGNES has been applied to indium solutions with either the hanging mercury drop electrode [33] or the thin film rotating disc electrode [34].…”

Back Accumulation of Diffusive Gradients in Thin-Films Devices with a Stack of Resin Discs To Assess Availability of Metal Cations to Biota in Natural Waters

“…The knowledge of relationships between lability degrees, sensor characteristics, and physicochemical parameters of the chemical species , opens the way to use the measurements of a set of sensors (equal or larger than the number of unknowns) to determine physicochemical parameters of the system as well as to predict the availability to microorganisms. In this case, a set of unknown kinetic dissociation constants and, accordingly, a set of DGT devices with different configurations in terms of thickness of the diffusive or the resin discs are required to write a set of equations determining the set of dissociation rate constants. ,,− Mobilities can also be included in the set of unknowns. Nonetheless, the detailed application of this procedure to natural waters is still unapproachable, unless a reduced number of pools of complexes sharing similar mobilities and labilities could be defined to simplify the system.…”

“…In this case, a set of unknown kinetic dissociation constants and, accordingly, a set of DGT devices with different configurations in terms of thickness of the diffusive or the resin discs are required to write a set of equations determining the set of dissociation rate constants. 17 , 18 , 23 − 25 Mobilities can also be included in the set of unknowns. Nonetheless, the detailed application of this procedure to natural waters is still unapproachable, unless a reduced number of pools of complexes sharing similar mobilities and labilities could be defined to simplify the system.…”

Availability of metals to DGT devices with different configurations. The case of sequential Ni complexation

“…Different types of analytical techniques have been developed to gain insight into the equilibrium and dynamic speciation of natural waters. Some of them are based on quantifying the concentration of the free ion in solution, such as Ion selective electrodes, Absence of Gradients and Nersntian Equilibrium Stripping (AGNES) or the Donnan Membrane Technique (DMT) (Bakker and Pretsch, 2007;Companys et al, 2018;Lao et al, 2018;Parat and Pinheiro, 2015;Temminghoff et al, 2000). Alternatively, other techniques provide a quantification of a labile fraction, which includes the contribution of the free ion plus an extra contribution of some complexes.…”